[CodeGen] Fix for PR39094.

When using MachineInstr to get SlotIndex, the MI could not be a debug
instruction. mi2iMap does not contain debug instructions in it.

After enabling DBG_LABEL in the generated code, the first instruction in
the bundle may be a debug instruction. In this patch, I use the first
non-debug instruction in the bundle to query SlotIndex in mi2iMap.

Bugzilla report: https://bugs.llvm.org/show_bug.cgi?id=39094

Differential revision: https://reviews.llvm.org/D52927

llvm-svn: 344770